Direct-contact membrane anode for use in electrolysis cells

ABSTRACT

The present invention relates to an anode system for conventional electrolysis cells, a process for the production thereof and its use for the deposition of electrolytic coatings. The anode system is characterized in that the anode ( 2 ) is in direct contact with a membrane ( 3 ) which completely separates the anode space from the cathode space. This anode system is therefore a direct-contact membrane anode.

The present invention relates to a membrane anode system for use in, for example, conventional electrolysis cells, a process for producing the membrane anode system and its use for the deposition of electrolytic coatings. This membrane anode system is characterized in that it works without an anolyte space and thus helps prevent undesirable anodic oxidation processes in electroplating electrolytes.

The electrochemical deposition of metals or metal alloys, referred to as coatings, on other metals or metal-coated plastics is an established technique for upgrading, decorating and increasing the resistance of surfaces (Praktische Galvanotechnik, Eugen G. Leuze Verlag). The electrochemical deposition of metals or metal alloys is usually carried out using anodes and cathodes which dip into an electrolysis cell filled with electrolyte. On application of an electric potential between these two electrodes (anode and cathode), metals or metal alloys are deposited on the substrate (cathode). In some cases, this construction is varied and an electrolysis cell in which the electrolyte is divided by means of a semipermeable membrane into a catholyte (electrolyte in the cathode space) and an anolyte (electrolyte in the anode space) is provided. The substrate (cathode) dips into the catholyte containing the metal ions to be deposited. On application of an electric potential, current flows via the anolyte through the membrane into the catholyte. These systems are already available on the market. However, an anolyte is used in the anode space in all these systems in order to ensure the flow of current. Maintenance, analysis and care of the anolyte require an additional outlay for these systems. If the membrane is damaged in these systems, anolyte gets in the cathode space and contaminates the catholyte. In most cases, the latter becomes unusable as a result and has to be disposed of in a costly fashion. A further disadvantage is the increased space requirement for the anolyte space, which is not available for the coating of goods. As a result, these conventional systems can be introduced into existing electrolysis cells only with a very high outlay in terms of cost and time, if at all.

In the case of many electrolytes which are operated using conventional, insoluble anodes, undesirable, anodic oxidation processes often occur. In these, the metals, complexing agents and organic additives present in the catholyte are oxidized at the surface of the insoluble anodes during deposition. As a result of these oxidation processes, the operating life of many electrolytes/electrolyte systems is limited since the oxidation products formed have an adverse effect on the electrolytic deposition. Furthermore, expensive, organic or inorganic compounds in the form of brighteners are destroyed at the anodes and have to be continuously replaced, which represents a large cost factor. In the case of noble metal electrolytes, an additionally large cost factor due to the necessity of fresh batches or time-consuming and costly purification processes arises.

The object of the present invention is, in particular, to avoid these undesirable anodic oxidation processes during deposition and to simplify existing membrane electrolysis systems so that the invention can be implemented directly in existing plants without costly modification work.

These and further objects which can be derived from the prior art in an obvious manner are achieved by an anode system which forms the subject matter of the present Claim 1. Particular embodiments of the anode system are defined in Claims 2-4. Claim 5 is directed to a suitable process for producing the anode systems of the invention. Claims 6-9 encompass the advantageous use of the anode systems of the invention in an electrolysis process. Claim 10 is directed to an electrolysis cell comprising the anode system of the invention.

The provision of an anode system which is configured in such a way that it is suitable for use in electrolysis cells for the deposition of electrolytic coatings as a result of simple dipping into the catholyte, wherein, after dipping into the catholyte, the catholyte is separated from the anode by a swollen polymer membrane which is permeable to cations or anions and the polymer membrane is in direct contact with the anode and not with the cathode, extremely advantageously but no less surprisingly achieves the stated object. The selected arrangement of the anode makes it possible to reduce the anolyte-filled anode region normally present in an electrolysis cell to the anode which is enclosed by a swollen polymer membrane and in contact therewith. This leads to the anolyte which is otherwise necessary for the electrolysis being able to be dispensed with entirely. Apart from the space-saving effect, it should also be noted that no undesirable oxidation reactions or damaging secondary reactions at the anode can now take place, which helps to increase the stability of the electrolyte significantly. Finally, it remains to be stated that the use costs for the electrolysis decrease since less material, in particular the additives described at the outset, is required. This was not to be expected in the light of the prior art. As regards the application of the polymer membrane to the anode, reference may be made to the information given in respect of the process of the invention for producing the direct-contact membrane anode.

As receptacle in which the electrolysis is carried out using the arrangement according to the invention, it is possible to use any vessel which comes into question for a person skilled in the art (Praktische Galvanotechnik, Eugen G. Leuze Verlag, fifth edition 1997, page 93 ff.). While the respective substrate on which the metals or metal alloys are to be deposited serves as cathode, the anode to be used in the present case is an insoluble electrode. Such anodes are adequately known to those skilled in the art. The anode can consist of flat material (flat material=metal sheets (metal anodes), plate material (GLC anodes, graphite anodes)), sintered material (http://de.wikipedia.org/wiki/Sintern) or expanded metal (supplied by Umicore Galvanotechnik GmbH, anodes for electroplating). As insoluble anodes, preference is given to using those composed of a material selected from the group consisting of platinised titanium, graphite, stainless steel, titanium coated with iridium-transition metal mixed oxide, tantalum or niobium and special carbon material (”Diamond Like Carbon” DLC, “glassy carbon” GC) and combinations of these anodes. Particular preference is given to mixed metal oxide anodes composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. Further suitable anodes may be found in Cobley, A. J. et al. (The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79(3), pp. 113 and 114). The shape of the anode can be matched appropriately to the respective electrolysis purpose by a person skilled in the art. Very particular preference is given to using a titanium, niobium or tantalum sheet coated with mixed metal oxides as insoluble anode material for the direct-contact membrane anode of the invention.

Possible polymer membranes which can be used for the purposes of the present invention are all membranes which come into question for this purpose for a person skilled in the art, e.g. cation- or anion-exchange membranes (ion-exchange membranes for short). In selecting the membranes, a person skilled in the art will firstly take account of the fact that only particular ions should be able to pass through the membrane and that the membrane should have a high uptake capacity for the process solutions so that the membrane is sufficiently conductive and, secondly, that the membrane should be capable of establishing direct contact with the anode ideally over the entire surface in order that optimal current transfer can actually take place. Suitable membranes for this application are all conventional ion-conducting membranes, known as ionomers. These are used routinely in polymer electrolyte membrane fuel cells or in batteries. Examples:

Polypyrrole membranes (Flintjer, B.; Jansen, W.: Polypyrrol und Polypyrrol-Batterien. In: Praxis der Naturwissenschaften-Chemie, Jg. 38, 1989, number 3, pp. 7-11.);

Olefin polymer membranes (DE19826702A1 17.12.1998, Verfahren zur Herstellung einer lonenaustauschermembran, die als Separator in einer Brennstoffzelle verwendbar ist, Solvay S. A., Brussels/Bruxelles, B E, Brunea, John A., Tavaux, F R), Example: TOPAS COC polymer membranes from TOPAS Advanced Polymers;

Sulphonated polystyrene membranes, perfluorinated ionomers (PFSI membranes), S-PEEK, S-PSU, PSU-CI, ICVT membranes (Bipolarplatten für Polymerelektrolyt Brennstoffzellen aus thermisch-und elektrisch hochleitfähigen thermoplastischen Kunststoffen, Rezeptierung, Herstellung, Charakterisierung und Anwendung; 2.4.1.4 Die Membran pages 30-33; Ralf Kaiser ISBN 978-3-8325-2033-5/Preparation of Membrane for Proton Exchange Membrane Fuel Cell, World

Academy of Science, Engineering and Technology 48 2008, Nilar Win, Mya Mya Oo);

Fluorinated/perfluorinated sulphonated polymer membranes (PFSA membranes).

EXAMPLES

-   -   Nafion membranes from DuPont Inc.: Nafion N112, Nafion N115,         Nafion N117, Nafion 324, Nafion N424, Nafion NR211, Nafion         NR212, Nafion N1110.     -   Aciplex membranes from Asahi Chemical Industry Company.     -   Flemion membranes from Asahi Glass Company.

R. Fernandez: Polymer Data Handbook, 1999, Oxford University Press, Inc., Pages 233ff);

Aryl polymer membranes (WIPO Patent Application WO/2001/064322, Application Number:PCT/EP2001/002311, Publication Date: 7 Sep. 2001, Filing Date: 1 Mar. 2001, Cui, Wei);

Polyether ketone membranes (Polymer electrolyte membrane and process for its manufacture. EP0574791, HOECHST AG, HELMER-METZMANN FREDDY DR);

Polybenzimidazole membranes (http://www.celanese.com/240501 powering the future-2.pdf);

Thermoplastic base polymer membranes (EP 0 698 300 B1 Polymer Brennstoffzelle, Fraunhofer-Gesellschaft zur Förderung der Angewandten Forschung EV 80636 Munich, Konstantin Ledjeff);

Perfluorosulphonic acid polymer membranes (OXIDATION-STABILISED POLYMER ELECTROLYTE MEMBRANE FOR FUEL CELLS, WIPO Patent Application WO/2008/025465, EP2007/007348, Publication Date: 6 Mar. 2008, DAIMLERCHRYSLER AG;

Perfluorocarboxylate ionomers (Flemion Asahi);

Polyamides, polyamines, poly(vinyl alcohol) membranes;

Perfluorophosphonate membranes;

Further suitable membranes are described in the book “Solid Polymer Electrolytes”, Wiley 1991, Fiona M. Gray.

Cation-permeable membranes are preferably employed. A particularly preferred polymer membrane is one selected from the group consisting of fluorinated/perfluorinated ionomers, very particularly preferably fluorinated/perfluorinated, sulphonated ionomers. These membranes have a high uptake capacity for electrolytes and therefore have a very low contact resistance, which significantly reduces electrolysis voltage. Further particularly preferred embodiments of these membranes are those reinforced with, optionally, Teflon fibres in order to achieve a high mechanical strength.

The present invention likewise encompasses a process for producing the direct-contact membrane anode of the invention. The process is characterized in that

i) the membrane is allowed to preswell in deionized water,

ii) the preswollen polymer membrane is applied directly to the anode and

iii) the latter is enclosed in the polymer membrane so that it cannot be wetted by the catholyte.

The direct contacting makes it possible to carry out an electrolysis in a relatively simple way by applying a current between cathode (substrate) and anode in a catholyte. A person skilled in the art will know how to allow the polymer membranes to preswell (e.g. manufacturer's information). The swelling of the membrane is preferably carried out in deionized, warm water which has been made slightly alkaline.

The application of the membrane to the anode is known to those skilled in the art from the field of fuel cell production (Handbook of Fuel Cells, Vol. 3, Wiley 2003, p. 538ff; http://www.fz-juelich.de/ief/ief-3/MEA_Herstellverfahren/). It is important that ideally direct contact with the anode is established, i.e. there must preferably be no gap between the membrane and the anode material. In the case of very close bonding between polymer membrane and anode, an advantageous flow of current is given, which results in a lower cell voltage. The membrane can be applied in the form of the polymer or in the form of a polymer solution to the anode. The preswollen polymer membrane can preferably be applied by lamination, pressing, adhesive bonding and/or clamping. If a polymer solution is used, the ion-selective layer is applied to the anode by dipping, casting, doctor blade coating, spraying, rolling and/or screen printing. The polymer solution can serve as final layer or as bonding agent between ion-exchange membrane and anode.

It is likewise important that the polymer membrane encloses the anode in such a way that no catholyte can get to the anode when it is dipped into the catholyte later, i.e. the anode is coated with the polymer membrane at all regions which are dipped into the catholyte. The layer thickness of the membrane can be determined by a person skilled in the art by means of routine experiments. A person skilled in the art will be guided by the fact that sufficient holding-back of the catholyte has to be brought into line with very optimal flow of current.

Overall, it is advantageous for the direct-contact membrane to be stabilized on the anode in any way in order to improve its mechanical durability and adhesion. This is achieved, for example, by the membrane being fixed onto the anode by means of electrolyte-permeable holders and pressing devices. This can occur, for example, by means of a multilayer structure on the membrane. The multilayer structure comprises a layer of sintered polymer. This layer ensures that the membrane is in contact with the anode over its entire area and prevents mechanical damage to the membrane. Perforated polymer plates are applied to this sintered layer. These plates press, the entire membrane-anode structure together and stabilize it. Good contact of the membrane with the anode is always ensured by this multilayer structure and at the same time protection against mechanical damage to the membrane is provided. A further possible way of stabilizing the membrane itself is to provide it with reinforcement by incorporation of support structures, e.g. Teflon fibres. As alternative support structures, monofilament or multifilament woven fabrics made of other inert materials are possible (www.fumatech.com/Startseite/Produkte/fumasep/Ionenaustauschermembranen/).

The present invention further provides for the use of the direct-contact membrane anodes of the invention for the electrochemical deposition of metal coatings on decorative and industrial articles. In the use according to the invention, the decorative and industrial articles are preferably dipped into the appropriate catholyte and a sufficient flow of current between these and the anode for deposition of the metal coatings is brought about. The further preferred embodiments mentioned in respect of the direct-contact membrane anodes or the process for producing them apply analogously to their use. The temperature during deposition is decisively determined by the catholyte used. In general, the temperatures at which the membrane still functions sufficiently well are in the range of 1-150° C., preferably 10-100° C. and very particularly preferably in a temperature range of 20-80° C.

The direct-contact membrane anode of the invention enables many metals and metal alloys to be deposited on appropriate substrates in normal plating cells. A person skilled in the art will select the electrolyte as a function of the metals and metal alloys to be deposited. Mention may be made by way of example of the following electrolytes which are suitable for the deposition of noble metals and base metals or metal alloys thereof selected from the group consisting of silver, gold, palladium, platinum, rhodium, ruthenium, iridum, rhenium, copper, tin, zinc, iron, nickel, cobalt, chromium, manganese, molybdenum, tungsten, tantalum, thallium, bismuth, antimony, indium, gallium, lead, cerium, selenium, cadmium, samarium, vanadium, tellurium and alloys thereof.

Particular preference is given to the general embodiment in which a titanium, tantalum or niobium sheet coated with mixed metal oxides (see above) is used as anode material. A membrane which has been preswollen in deionized water is then pressed onto this metal sheet so as to enclose it fully so that the region of the anode which dips into the catholyte is not wetted by the latter. As an alternative, clamping-on is likewise possible. This anode is subsequently dipped into a catholyte and a flow of current between cathode (substrate) and anode is brought about.

The present invention likewise provides an electrolysis cell having a cathode, a catholyte and an anode system according to the invention as described above for the deposition of electrolytic coatings. Preferred embodiments of the electrolysis cell may be derived from the passages above relating to the anode system.

As advantageous field of application, the direct-contact membrane anodes can be employed in the coating of components in acidic copper electrolytes. In the conventional coating process, soluble anodes are used. As a result of the anodic current yield being higher than the cathodic current yield, copper is concentrated in these electrolytes. An acidic copper electrolyte therefore has to be diluted every now and again in order for the operating parameters not to be shifted. Normally, insoluble MMO anodes (mixed metal oxide anodes) are additionally introduced into the electrolyte in order to reduce the soluble anode area so that concentration of copper in these electrolytes does not occur. However, the organic brighteners are destroyed by these anodes and a very large loss of expensive, organic additives occurs. If the soluble anodes are combined with the direct-contact membrane anodes, the anodic oxidation of these expensive additives is avoided.

A further field of application is the use of the direct-contact membrane anodes in the deposition of chromium from Cr(III)-containing electrolytes. In these electrolyte systems, oxidation of the chromium to chromium(VI) occurs at insoluble anodes. This oxidation state of chromium is very toxic and carcinogenic and the electrolyte becomes unusable. As a result of the use of direct-contact membrane anodes, this oxidation is avoided and occupational hygiene is thereby improved and the life of the electrolyte is increased by a multiple.

In the conventional processes for the deposition of palladium-nickel, insoluble anodes are used. With time, the complexing agent which is always present is destroyed at these anodes and the Pd is converted into a higher oxidation state. As a result, the deposition rate in this electrolyte decreases after only a short time (0.5 Ah/l) and the deposited layers become unusable. The conventional method of restoring this electrolyte to the initial deposition performance is treatment with activated carbon.

However, this is time-consuming and costly. When the direct-contact membrane anode is used in palladium-nickel electrolytes, these oxidation processes are avoided and the electrolyte has a significantly longer operating life (>20 Ah/l).

Further electrolytic deposition processes likewise profit from the use of the present anode system. Furthermore, the novel anode system is easier to handle than conventional systems in which there are separate cathode and anode spaces. The system can be implemented in existing electroplating plants without great modification work. Apart from a current yield of almost 100%, the direct-contact membrane anode of the invention brings about a significantly increased operating life of electrolyte systems (see above) due to the reduced oxidative destruction of additives and the absence of secondary reactions at the anode. This was not to have been expected in the light of the available prior art.

FIGURES

FIG. 1—Structure of a conventional membrane cell

FIG. 2—Structure of direct-contact membrane anode

FIG. 3—Structure of an electrolysis cell with direct-contact membrane anodes

REFERENCE NUMERALS

1 Cathode

2 Anode

3 Ion-exchange membrane

4 Anolyte

5 Catholyte

6 Electrolysis vessel

7 Mechanical protection against damage

8 Direct-contact membrane anode 

1. Anode system which is configured in such a way that it is suitable for use in electroplating cells for the deposition of electrolytic coatings as a result of simple dipping into the catholyte, wherein, after dipping into the catholyte, the catholyte is separated from the anode by a swollen polymer membrane which is permeable to cations or anions and the polymer membrane is in direct contact with the anode and not with the cathode, wherein the membrane is fixed onto the anode by means of electrolyte-permeable holders and pressing devices by means of a multilayer structure, which ensures good contact of the membrane with the anode.
 2. Anode system according to claim 1, wherein. the anode consists of flat material, sintered material or expanded metal.
 3. Anode system according to claim 1 wherein the anode is selected from the group consisting of platinised titanium, graphite, stainless steel, titanium coated with iridium-transition metal mixed oxide, tantalum or niobium sheets coated with mixed oxides and special carbon material (“Diamond Like Carbon” DLC, “glassy carbon” GC) and combinations of these materials.
 4. Anode system according to claim 1, wherein a membrane selected from the group consisting of polypyrrole membranes, olefin polymer membranes, sulphonated polystyrene membranes, fluorinated/perfluorinated sulphonated polymer membranes (PFSA membranes), S-PEEK, S-PSU, PSU-CI, ICVT membranes, aryl polymer membranes, polyether ketone membranes, polybenzimidazole membranes, thermoplastic base polymer membranes, perfluorosulphonic acid polymer membranes, perfluorocarboxylate ionomers, polyamides, polyamines, poly(vinyl alcohol)membranes, perfluorophosphonate membranes is used as the polymer membrane.
 5. Process for producing an anode system according to claim 1, wherein i) the membrane is allowed to preswell in deionized water, ii) the preswollen polymer membrane is applied directly to the anode, iii) the membrane is fixed onto the anode by means of electrolyte-permeable holders and pressing devices by means of a multilayer structure, which ensures good contact of the membrane with the anode, and iv) the anode is enclosed in the polymer membrane so that it cannot be wetted by the catholyte.
 6. A process for the electrochemical deposition of metal coatings on decorative and industrial articles, comprising utilization of the anode of claim 1 in a catholyte.
 7. The process according to claim 6, wherein an article is dipped into the catholyte and a sufficient flow of current between the article and the anode for deposition of the metal coatings is brought about.
 8. The process according to claim 6, wherein the deposition is carried out in a temperature range of 1-150° C.
 9. The process according to claim 6, wherein a catholyte capable of depositing metals or metal alloys selected from the group consisting of silver, gold, palladium, platinum, rhodium, ruthenium, iridum, rhenium, copper, tin, zinc, iron, nickel, cobalt, chromium, manganese, molybdenum, tungsten, tantalum, thallium, bismuth, antimony, indium, gallium, lead, cerium, selenium, cadmium, samarium, vanadium, tellurium and alloys thereof is selected as the catholyte.
 10. An electrolysis cell comprising a cathode, a catholyte and an anode system according to claim 1 for the deposition of electrolytic coatings. 